I found that on ?-8 that you mostly teleport back to the far left or far right. So on my second go I made sure to stack up my squads (due to being under leveled) to make sure I took out the middle groups of enemies/bosses. After that it was just a run around, teleport back, take everything else out at random. Two of the bosses hit like trucks for me. The one from the second from left camp, and the final one. Luckily, I had a strong supporter who was the last one left by the end.

Happy to have cleared this event, as it's the first one dealing with medals that I have.